This invention relates to a method of brazing aluminium or aluminium alloy parts, and more particularly the method of brazing them with a flux.
The invention also relates to an atmosphere furnace which can advantageously be employed for such brazing method.
When iron, copper, or their alloys are put into a furnace employing as its atmosphere a reductive gas such as hydrogen and carbon monoxide, and are subjected to a high temperature, they are deoxidized. Therefore, the brazing of such metals within such atmosphere furnace does not need a flux.
On the other hand, as aluminium and its alloys are not deoxidized even under an extremely low partial pressure of oxygen, they can not be brazed by heating them within the atmosphere furnace of the kind mentioned above. Accordingly, their brazing has been made not by an atmosphere but by the use of a flux.
Conventional brazing may be summarized that the brazing of iron, copper, or their alloys by heating them within a furnace employs a reductive atmosphere but does not need a flux, while the brazing of aluminium and its alloys does employ a flux but does not need a reductive atmosphere.
On the contrary to the above-summarized conventional brazing methods, it becomes lately useful or essential for brazing, with a flux, aluminium and its alloys to employ an inert atmosphere, too. This is in order to braze aluminium parts with a minimum amount of flux and effectively without causing environmental pollution.
To wit, as those fluxes which are commonly used for brazing aluminium parts and have a melting point of 500-550.degree. C., are made from fluorine compounds such as aluminium fluoride, potassium tetrafluoroaluminate, sodium fluoride, and so on, or alkali metal chloride containing one or more fluorine compounds, they are soluble only slightly in water, and accordingly it needs a large volume of water to wash off them. In order to reduce the volume of water for washing off them or to eliminate a washing step in view of environmental pollution, they must be used as minimum as possible.
While the reduction of use of fluxes can be against environ-mental pollution, especially water pollution, it can be economic too. It can yield fine aluminium articles with little flux residues. It can prevent a furnace from being eroded much by fluxes.
However, fluxes which have been applied over aluminium parts thinly in order to minimize them, are readily oxidized at a high temperature. Oxidized fluxes of the kind mentioned above will have a melting point more than 1,000.degree. C. Since a melting point of Al--Si system solders which are commonly used in aluminium brazing, is about 500-630.degree. C., and since the aluminium or aluminium alloy brazing is conducted at about 580-660.degree. C., the oxidized fluxes having the melting point as high as 1,000.degree. C. are unserviceable. They will induce secondary oxidation of aluminium or aluminium alloy parts, to which they have been applied.
Consequently, even in the brazing of aluminium or aluminium alloy parts, heating them under an inert atmosphere becomes essential, as described above, in order to prevent fluxes from oxidizing. In practice, a hermetic metallic muffle filled up with nitrogen gas of a high purity is installed in a furnace, and the aluminium or aluminium alloy parts are brazed with a flux under an inert atmosphere within the metallic muffle.
While such metallic muffle protects its atmosphere from an outer disturbance, electric heating elements which are installed in a space between the muffle and inner furnace walls, are protected by the muffle from fluxes scattered therein, whereby the muffle prevents the heating elements from troubling electrically on account of the fluxes. Although metallic muffles work so, it is not easy to replace them when they are damaged by fluxes and so on. As metallic muffles employed today are as long as 10 m, it is laborious to install or replace them.
It is therefore an object of this invention to provide a method of continuously brazing aluminium or aluminium alloy parts with the use of fluxes and under an inert atmosphere such as nitrogen within a furnace which does not employ any metallic muffle, and to provide also the furnace which can carry out the method well.